The electrical grid supplies electricity to consumers connected to the grid for various types of electrical load, for example: electronics; lighting; heating, ventilation and air condition; and transportation equipment. Each electrical load requiring electrical energy from an electrical grid in the form of alternating current and voltage can have time varying load demands for electrical power either in the long term (for example, month-to-month) or in the short term (for example, minute-to-minute). Also the electrical load may be stable or unstable.
Often electrical loads on the transmission and distribution portions of an electrical grid have a reactive power (Q) demand measured in volt-amperes reactive (var) in addition to an active (or real) power (P) demand measured in watts (W) that can be mathematically represented as a total complex power, S, measured in volt-amperes (VA) by the equation:S=P+jQ   (Equation 1)where j is the mathematical imaginary unit.
The electrical grid must supply the magnitude of complex power, S, as demanded by the connected electrical loads.
The current for the required active power demand is vectorially in phase with the applied voltage and active power (W) produces electrical (mechanical) work (for example; a motor rotating a conveyor belt; compressors heating or cooling an enclosed volume; or lighting fixtures supplying light to an area). Active power demand magnitude can be mathematically expressed by the equation:P=S×cos ø  (Equation 2)where cos ø is the power factor, which is equal to the ratio of active power (P) to complex power (S).
The current for the required reactive power demand does not produce work but is required for the electrical characteristics of certain loads and burdens the electrical grid with reactive current requirements that vectorially lag the applied voltage by 90 degrees. Reactive power demand magnitude can be mathematically expressed as:Q=S×sin ø  (Equation 3).
Electrical grid operators monitor transmission and distribution circuits connected to the grid for variations in the power factor, load magnitude, voltage, and frequency. Excessive reactive power in the connected circuits affects not only the efficiency but also the stability of the overall grid (power system).
Excessive reactive load currents are carried by power lines between the grid's generating sources and loads connected to the grid without producing work and can cause sagging line voltages and load voltages. Reactive load power can optionally be generated or absorbed in close proximity to a load to eliminate reactive currents from the power lines.
Similarly, varying load demand, as well as reactive load currents, cause voltages on the grid power lines and load to fluctuate. Power distribution regulators obligate the operator of the grid to maintain strict limits on the supplied voltage, typically no greater than five percent or less than ten percent of a nominal grid voltage. The grid frequency is controlled more strictly to no more than 0.02 percent or less than 0.02 percent of a nominal value by some grid regulators. To mitigate the effects of varying load demand and reactive loads electrical utilities employ different devices and procedures.
Grid power generation and power consumption should always be balanced. Surplus grid power generation greater than grid power demand causes increase in voltages and frequency on the grid. Conversely, deficit grid power generation less than grid power demand causes decreases in voltages and frequency on the grid.
While reduction of grid voltage in some degree has a self-stabilizing effect on linear loads (that is, power consumption drops as voltage drops), grid frequency reduction in sections of the grid network can damage the grid by allowing grid power generators to loose synchronization and become unstable. For this reason the regulations on frequency variations are stricter as mentioned above, and time for recovery from a grid low frequency condition is shorter.
Reactive loads can be compensated for by passive reactive components. To some degree increasing load demand may be compensated for by increased energy supply. Other means of stabilizing the voltage may be utilized, such as: increased tariffs during times of high demand; penalties to grid customers with large inductive loads (that is, low power factor loads); curtailment of power generation during periods of low demand, or in extreme cases, disconnecting grid customers during periods of high demand on the grid.
When load is changing rapidly it is often very difficult to react quickly and maintain stable voltages on the distribution network of the grid, particularly when passive components, such as capacitor banks, are used since they have limited response times as well as a limited number of engagement cycles that prevent the passive components from being actively used on a large scale to an electrical grid.
Renewable electrical energy sources, such as photovoltaic or wind energy sources, could play a positive role in grid stabilization. Solar energy typically coincides with periods of high power demand on the grid. However renewable electrical energy sources can introduce a degree of instability due to the intermittency of the supply of electrical power from solar energy depending upon geographical location of the sources. Combinations of renewable energy sources (for example, wind and solar) and energy storage can alleviate the intermittency issue.
Power electronics used in renewable energy and storage systems generally employ direct current to alternating current (DC/AC) inverters that can provide short time response capabilities far greater than conventional rotating generators when it comes to power generation. This remains true when compared to most conventional means of compensating for reactive power. In many cases these power electronics can provide such capabilities without any changes in hardware, and therefore can help provide power support to a grid in an economical fashion.
Most DC/AC inverters used in renewable energy power generation and power factor correction applications utilize pulse switch modulation technology (PWM). However PWM switching typically results in high inverter losses which prevent the generation of substantial reactive power. Also the smoothing inductors both in the input of the active filter and the output of the inverters also contribute to the inductive characteristics of the load.
U.S. Pat. Nos. 8,130,518 B2 and 8,213,199 B2, which are incorporated herein by reference in their entireties, disclose the concept of multiphase grid synchronized regulated current source inverter systems (MGS-RCSIS) where the combination of a plurality of current fed three phase inverters convert DC voltage from solar and wind renewable energy sources into AC voltage compatible with the electrical grid. Conversion is accomplished by current regulation of the output from the DC renewable energy sources to the input of each one of multiple inverters in a MGS-RCSIS system, with each inverter outputting multiple phase currents that are out of phase with the multiple phase currents outputted from all other inverters in the system. Each of the multiple phases has a step-shape current from all of the inverters in a system that are connected to the secondary windings of a phase transformation network that produces a three phase sine wave current output waveform for injection into the electric power grid. The method of reduction of harmonic distortion by combining a plurality of phase-shifted distorted currents from a number of inverters is known as the harmonic neutralization (HN) method.
It is one objective of the present invention to provide a co-located multi-mode, large scale electric power supply support system for an electrical grid where the support system's power source is solar or wind renewable energy with a minimum capacity of 2,500 kW integrated with a stored energy power source where the renewable energy power can be supplied totally or partially to the grid with low harmonic distortion in combination with supply of stored energy power to the grid or charging the stored energy power source from the renewable energy power source or the grid.
It is another objective of the present invention to provide a co-located multi-mode, large scale electric power supply support system for an electrical grid where the support system's power source is solar or wind renewable energy with a minimum capacity of 2,500 kW integrated with a stored energy power source where the electric power support system can absorb reactive power from the grid or deliver reactive power to the grid to improve grid stability.